1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and, more particularly, to a method for manufacturing a semiconductor device including forming an interconnect by damascene process in a porous insulating film.
2. Related Art
Recently, with the remarkable high speed of operations of semiconductor devices, there has been a problem that transmission delay is caused by reduction in a signal propagation speed due to interconnect resistance in a multilayered interconnection structure and parasitic capacitance between interconnects. Such a problem has been tending to be more and more remarkable because the interconnect resistance increases, and the parasitic capacitance between interconnects increases as the width of the interconnect and the interconnect interval become finer along with higher levels of integration of the semiconductor devices. Then, in order to prevent the signal delay due to the interconnect resistance and the parasitic capacity between interconnects, a copper interconnect, instead of a conventional aluminum interconnect, has been introduced. At the same time, it has been tried to use a low dielectric constant insulating film, or a low-k film, for the interlayer insulating film. Here, the low dielectric constant film is an insulating film with a relative dielectric constant not larger than 3.9, which is the relative constant of a silicon dioxide (SiO2).
One of the methods for forming the above-described copper interconnect includes a dual/single damascene process. Considering that control of an etching rate for copper is more difficult in comparison with that for aluminum, the copper interconnect is formed by the damascene process which is a technology by which interconnect is formed without etching process. The damascene process includes forming an interconnect trench or a via hole in an interlayer insulating film and filling the interconnect trench or the via hole with copper or copper alloy. A multilayered copper interconnect structure is formed as follows, for example. Firstly, a silicon carbide (SiC) film with a small relative dielectric constant as an insulated barrier layer (or an etching stopper layer) is formed on the lower copper interconnect. Then, an organic insulating film as a low-k film and an SiO2 film as a cap film are deposited in this order on the silicon carbide film to form a laminated film. Thereafter, a via hole or interconnect trench of a dual damascene process is formed in the laminated film by dry etching thereof. Then, an interconnect material comprising a barrier metal, and copper or copper alloy material is formed in the via hole or the interconnect trench to form a via plug or copper interconnect in the upper layer. The interconnect material is formed by a sputtering method, an atomic layer deposition (ALD) method, a copper plate method, and the like in such a way that the interconnect material fills the via hole or the interconnect trench. Then, the interconnect material is selectively removed by a chemical mechanical polishing (CMP) method so that the interconnect material is left only in the via hole or the interconnect trench and the surface of the interconnect material is made flatter.
In development of a semiconductor device, finer component elements have been developed, for example, the design rule has changed to 45 nanometers from 65 nanometers. Moreover, recently, the relative dielectric constant of a low dielectric constant film used for the damascene interconnect is required to be about 2.0 or less. Thereby, a porous low dielectric constant film has been indispensable as a low-k film.
However, when the via plug or the copper interconnect, formed by the damascene process, is put into practical use, with the low-k film used as the interlayer insulating film, the following problems are generated.
Firstly, the surface of the lower copper interconnect which is exposed when the via hole or the interconnect trench is formed in the upper laminated film by dry etching, is easily oxidized. Thus, an insulating reactant adheres to the surface. At the same time, the side wall of the via hole or the interconnect trench, formed in the above-described laminated film easily undergoes side-etching to cause the manufacturing yield of the semiconductor device having the above-described copper interconnect lowered. Here, the adhesion of the above-described insulating reactant causes poor electric connection of the copper interconnect between the lower or the upper interconnect. Moreover, the side etching of the above-described side wall causes bad filling of the interconnect material in the via hole or the interconnect trench.
Therefore, the etching gas used for dry etching of the above-described laminated film, especially, the SiC film of the insulating barrier layer has been variously examined, and it has been proposed that a nitrogen content gas, instead of an oxygen content gas, is added to the etching gas (for example, Japanese Laid-open patent publication No. 2003-124200 and Japanese Laid-open patent publication No. 2002-110644).
Secondly, especially when a porous film with a low dielectric constant is used for the low-k film, a lot of holes (pores) are exposed on the side wall of the above-described via hole or the interconnect trench during the manufacturing process. Then, moisture, copper of the interconnect material, or the barrier metal such as tantalum (Ta), or tantalum nitride (TaN) intrude into the inside of the low-k film through the holes or pores to cause reduction in the reliability of the interlayer insulating film, rise in the relative dielectric constant, increase in the leakage current between the interconnects, and the like.
Therefore, it has been proposed (for example, Japanese Laid-open patent publication No, 2003-197742 and Japanese Laid-open patent publication No. 2000-294634) that an inorganic insulating film (pore seal) with a close-grained film quality is provided on the side wall of the above-described via hole or the interconnect trench as a side-wall protection film.
However, the above-described conventional side wall protection film includes an insulating film such as an SiO2 film with a relative dielectric constant of about four, or a metallic oxide layer with a larger relative dielectric constant than that of the SiO2 film to have an extremely higher relative dielectric constant than that of the low dielectric constant film with a relative dielectric constant of about 2.5 or less. Accordingly, when the above-described conventional side-wall protection film is applied to formation of the copper interconnect by the damascene process, the relative dielectric constant of the whole interlayer insulating film increases, and it becomes difficult to reduce the parasitic capacity between the copper interconnects.